Accuracy and Time To First Fix (TTFF) are critical parameters for any Global Positioning System (GPS) location based service. TTFF is the measure of time required for a GPS receiver to acquire satellite signals and navigation data, and calculate a position solution, which is called a “fix,” a position derived from measuring external reference points.
In a traditional GPS system, the GPS receiver must find the satellites in order to establish a location fix. The satellites and generally, the GPS receiver are in motion, thus there is finite shelf life for the satellite constellation data. Ephemeris and almanac data provide clues as to where the satellites should be for a short term (ephemeris) and longer term (almanac). This data is transmitted by the satellites. However, this data is a very low baud rate, and is broadcast only—there is no handshake. Thus, a GPS receiver must wait until data is broadcast, it cannot query when the data is needed. Thus, it may take a GPS receiver a long time to achieve a fix.
GPS navigation satellites transmit electronic ephemeris data consisting of health and exact location data that GPS receivers then use (together with the signal's elapsed travel time to the receiver) to calculate their own location on Earth using trilateration. This ephemeris data is broadcast from the satellites and is used to locate the satellite. But collection of the ephemeris data from the satellite can be slow and problematic.
An alternative to gathering the ephemeris data locally is to create an almanac of the satellite position(s) and send this data to the GPS receiver. The major drawback to this approach is obtaining the almanac data and delivering the data to the GPS receiver.
Another issue with traditional almanac data is the shelf-life of the data. GPS satellites tend to drift in orbit and experience slight changes in position based on a number of external factors. Thus, almanac data is valid for a maximum of 14 days.
A need therefore exists for a way to increase the speed and accuracy of location-based services using GPS by utilizing ephemeris data more effectively.